Thermal Management In Circuit Board Assemblies

ABSTRACT

Vias may be established in printed circuit boards or similar structures and filled with a monolithic metal body to promote heat transfer. Metal nanoparticle paste compositions may provide a ready avenue for filling the vias and consolidating the metal nanoparticles under mild conditions to form each monolithic metal body. The monolithic metal body within each via can be placed in thermal contact with one or more heat sinks to promote heat transfer.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of priority under 35 U.S.C. §119 from U.S. Provisional Patent Application 62/625,668, filed on Feb.2, 2018 and incorporated herein by reference in its entirety.

BACKGROUND

Ineffective thermal communication between a heat source and a thermalmanagement system like a heat sink can hamper dissipation of excess heatfrom a system. Electronic components, such as high-power LEDs andhigh-power circuitry, for example, are continually decreasing in sizeand becoming ever more powerful, thereby generating loads of excess heatthat are increasingly being concentrated in smaller and smaller spaces.Growing production of excess heat and concentration thereof can makeeffective heat removal especially problematic.

Failure to remove excess heat from an electronic system can result insignificant consequences such as, for example, overheating, reducedconduction, higher power requirements than normal, and/or the need forclock-down operation to avoid board burnout and device failure. In manyinstances, operational modifications are employed to limit theproduction of excess heat, rather than altering the system architectureto promote better heat removal. Thus, certain systems may be operated inan inefficient condition compared to how they would otherwise beoperated if removal of the excess heat did not prove so problematic.

Ineffective heat conduction can be especially prevalent in circuitboards of various types, particularly printed circuit boards (PCBs).PCBs and similar circuit boards are thermal insulators by the verynature of their construction. Specifically, PCBs generally employthermally insulating substrates (e.g., glass fiber epoxy composites likeFR4, which has a thermal conductivity value of 0.25 W/m·K), upon whichappropriate electronic circuitry and various board components aredisposed. The low thermal conductivity values of PCB substrates can makeremoval of excess heat from electronic systems rather difficult. Verylittle excess heat is capable of being removed via the leads due totheir typically small size. In addition, conventional lead solder is notespecially thermally conductive (e.g., about 1/10^(th) or less than thatof more thermally conductive metals, such as copper). Similarly, themetal traces (circuitry) defined in PCBs are typically thin and embeddedin the substrate, thereby not allowing much heat dissipation to takeplace.

In some cases, heat dissipation may be improved by adding a thermalground plane to one face of the PCB. However, this design prevents theuse of that particular face of the board for adding components toincrease the complexity and functionality of the PCB, thereby limitingits design and use. Another option sometimes used is incorporation of athicker copper layer in the center of the board as a highly conductivethermal path. This approach poses manufacturing challenges toincorporate such a large and thick copper sheet, particularly due to itsvastly different thermal expansion properties, thereby leading tomechanical stresses during PCB manufacture and operation. In fact, themechanical stress resulting from thermal expansion may render the PCBineffective for the very thermal conduction purpose it was designed for.Again, the complexity of the board may be limited unless the board canbe effectively connected to a thermal plane.

Current architectures for removing excess heat from PCBs or similarcircuit boards only offer one way out for the excess heat, through aheat sink or similar thermal management device in thermal communicationwith a component generating the excess heat, such that the excess heatis conveyed away without passing through the PCB substrate. Thisapproach allows heat dissipation to take place from only a single faceof the PCB. Growing heat removal demands have often necessitated the useof increasingly large and heavy heat sinks, such as machined copperblocks, to maximize the surface area in thermal communication with thesource of excess heat to promote better heat dissipation. Even so, itcan still be difficult to dissipate excess heat due to the poor thermalconductivity of the PCB substrate and limited available space formounting the heat sink. Moreover, copper and similar metal blocks areheavy, which may be undesirable for payload-sensitive operations, andthey may be subject to disengagement during rough transport or otherconditions of use, thereby precluding effective conveyance of excessheat. Thus, present approaches for affecting removal of excess heat fromPCBs and similar circuit board assemblies are becoming increasingly lesseffective, thereby hampering further advances in board technology as awhole.

It is possible, in principle, to introduce features into PCBs or similarcircuit board assemblies to improve heat conduction, including to theopposite face of the board relative to where the excess heat is beinggenerated, but this approach can lead to problems in its own right usingconventional metal processing techniques. For example, one or more holes(vias) may be drilled through a PCB substrate directly beneath acomponent producing excess heat, and the holes may be loaded with ahighly conductive material, such as copper. This approach could increasethe overall thermal conductivity value of the PCB. However, directliquid casting of metals into vias is not compatible with the boardmaterials that are presently in use (metal processingtemperatures >1000° C. in comparison to much lower polymer meltingpoints for materials typically used as substrates). As such, vias areoften packed with rosin or a similar filler and then galvanically cappedat the ends or left open with just a thick metal plating (e.g., copper)on the via walls (i.e., the via barrel). Coating thicknesses upon thevia walls are typically in the range of ˜25 microns. Although metalplating approaches may be effective to promote electronic conductivitybetween components in various board layers, metal plating onlymarginally increases the thermal conductivity profile, since the surfacearea of the metal exposed at the board face remains rather small.

As such, thermal vias of the foregoing type are usually limited to smalldiameters, oftentimes a diameter of 1 mm or less, because electrolesscopper plating does not allow for effective filling of large holes orhigh aspect ratio vias. Since it would take a very long time (days) tocompletely fill vias having a diameter in this size range, a largenumber of incompletely filled vias are often used for promotingeffective heat removal. However, this approach may be insufficient forremoving large quantities of heat (e.g., >100 W/cm²). Moreover, anexcessive amount of vias can promote mechanical weakness in a PCB.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are included to illustrate certain aspects of thepresent disclosure, and should not be viewed as exclusive embodiments.The subject matter disclosed is capable of considerable modifications,alterations, combinations, and equivalents in form and function, as willoccur to one having ordinary skill in the art and the benefit of thisdisclosure.

FIGS. 1 and 2 show diagrams of presumed structures of metalnanoparticles having a surfactant coating thereon.

FIGS. 3A-3D show side-view cross-sectional diagrams illustrating variousoperations associated with filling vias with a metal nanoparticle pastecomposition and forming a monolithic metal body therein.

FIG. 4 shows a top-view diagram of a printed circuit board havingmetal-filled vias extending therethrough.

FIG. 5 shows a stress-strain curve for consolidated copper nanoparticlesin comparison to other materials frequently used in conjunction withprinted circuit board manufacturing.

FIG. 6 shows an SEM image that illustrates porosity in a consolidatedcopper nanoparticle paste composition.

DETAILED DESCRIPTION

The present disclosure is generally directed to circuit board technologyand, more specifically, to techniques for thermal management in printedcircuit boards (PCBs) and related electronic systems. Other systems inwhich dissipation of excess heat is problematic may also be addressedeffectively with concepts related to those disclosed herein.

As discussed above, removal of excess heat from circuit boards andrelated electronics assemblies can be problematic due to the prevalenceof thermally insulating materials therein. Because of this issue,effective thermal management techniques presently utilize rather largeheat sinks, with heat removal taking place from only one face of thecircuit board. As such, limited amounts of excess heat can be removedwhen using conventional thermal management techniques.

Although metals may be electrodeposited onto via walls to promoteelectronic conductivity between components in various board layers, thethin metal layers do little to increase thermal conductivity forpromoting dissipation of excess heat. It is very problematic indeed toextend metal plating approaches to afford more complete filling of viasto improve thermal conductivity. Via filling with electrodeposited metal(e.g., copper) usually takes many hours to complete and must beperformed carefully to avoid premature capping of the hole and formationof an excessive void space in the via, thereby leading to even poorerthermal conduction. Electrodeposition approaches are therefore veryexpensive and are often a money-losing operation for manufacturers.Moreover, vias that can be successfully filled via electrodepositiontypically have very narrow cross-sectional profiles (diameters) of about100 microns to about 300 microns in order to limit the time needed tocomplete the electrodeposition process and obtain a fill factorapproaching 100%. The small cross-sectional profiles ofelectrodeposition-filled vias again limits the amount of thermalconduction that may take place. In addition, for the most effectivethermal management benefits to be realized, the vias are typicallylocated directly beneath a board component producing the excess heat. Itcan be difficult in its own right to introduce and fill vias in thislocation, particularly if the board component has already been placed onthe PCB.

The present disclosure provides an entirely different approach forfilling vias and similar conduits to establish one or more thermalconduction pathways extending completely through the insulatingsubstrate of a PCB or similar circuit board assembly. Advantageously,the via-filling approaches described herein may be performed rapidly andat much lower cost than electrodeposition approaches, while stillreadily affording high-quality, large thermally conductive monolithicmetal bodies that extend completely through a PCB substrate. Thethermally conductive areas may be in the form of trenches, ducts, and/orvias. Thus, although the present disclosure is largely directed to thefilling of vias, it is to be appreciated that alternative structures maybe processed analogously. Because the various approaches of the presentdisclosure allow relatively large vias or similar conduits to be filledwith metal, a greater degree of thermal conduction may be realizedcompared to that obtainable conventionally using electrodepositiontechniques.

Advantageously, the present disclosure provides a PCB architecture fromwhich excess heat can be dissipated from a face of the board oppositethat where a component producing excess heat is located. Namely,metal-filled vias produced according to the disclosure herein may allowexcess heat to be conveyed from a first face of the board to a secondface of the board (i.e., across the insulating substrate of the board),where the excess heat may be more effectively dissipated in some cases.For example, it may be easier to locate a thermal management device on aface of the PCB opposite a component producing the excess heat.Moreover, the present disclosure also allows heat dissipation to takeplace from both (opposing) faces of the board, if desired, therebyallowing the same or different thermal management approaches to beutilized upon each face. Furthermore, in some embodiments, the presentdisclosure allows for a thermal connection to be established between aface of the board and a metal casing housing the board, thereby allowingthe metal casing to function advantageously as a very large heat sinkwithout utilizing a dedicated metal block to promote thermal conduction.In some cases, a direct metallurgical bond may be established between ametal-filled via and a thermal management device in contact therewith.

More specifically, the present disclosure utilizes metal nanoparticlecompositions to affect filling of vias or similar structures forestablishing one or more thermally conductive pathways extending throughthe insulating substrate of a PCB or similar circuit board assembly. Thethermally conductive pathway formed from the metal nanoparticles maycomprise a monolithic block of metal or metal composite (equivalentlyreferred to herein as a “monolithic metal block” or “monolithic metalbody”) extending between the first and second faces of the PCB. Themonolithic metal block may span the entire cross-sectional profile ofthe via, thereby providing a much larger exposed surface area forpromoting thermal conduction than is possible with electrodepositionapproaches. As discussed above, electrodeposition approaches aredifficult to perform and are at considerable risk of incomplete viafilling and/or void formation, thereby leading to poor thermalconduction. Such issues are much less prevalent when utilizing metalnanoparticles, thereby allowing vias having a much largercross-sectional profile to be filled with metal.

Previous approaches for filling vias with metal nanoparticles aredescribed in U.S. Patent Application Publication 2015/0114707. However,there may be difficulties associated with effective filling, cracking,delamination, and failure, particularly for vias above a particularsize.

Metal nanoparticles are uniquely qualified for the applicationsdescribed herein due to the moderate processing conditions needed forconsolidating the metal nanoparticles together into a monolithic blockof the corresponding bulk metal. Namely, as described in further detailbelow, metal nanoparticles can be consolidated (fused) together into thecorresponding bulk metal under a range of mild processing conditionsthat are significantly below the melting point of the metal itself. Dueto copper's high thermal conductivity and relatively low cost, coppernanoparticles can be a particularly desirable type of metal nanoparticlefor use in the various embodiments of the present disclosure.

In addition, if desired or needed, metal nanoparticle compositions maybe further tailored to improve the thermal conductivity still furtherwhen filling vias or similar structures. In particular, the metalnanoparticle compositions may be tailored to limit shrinkage duringfusion, which may exceed 20% in other metal nanoparticle systems, andlimit thermal expansion to reduce thermomechanical stress duringoperational hot-cold cycling. More specifically, metal nanoparticlecompositions suitable for use in the present disclosure may containlarger micron-sized, highly thermally conductive particles (e.g.,copper, diamond, carbon nanotubes, graphene, and the like) while stillbeing easily dispensed by various via-filling techniques. These featuresgreatly simplify PCB assembly and provide overall product cost reductionwhile significantly enhancing performance. Once nanoparticleconsolidation occurs, the resulting bulk metal binds the micron-scaleadditives together to form a monolithic metal body extending through theinsulating substrate of the PCB, thereby providing a thermallyconductive pathway through the substrate. Additional tailoring topromote dispensation and/or consolidation of the metal nanoparticles mayalso be performed, as described hereinafter.

Although advantageous for promoting thermal conduction, the concepts ofthe present disclosure can also be employed for general filling of viasas well, more specifically buried vias and thermal traces within theboard architecture, thereby allowing single-step co-processing to berealized during various board lamination steps to allow readyincorporation into the overall PCB manufacturing process. Through-planevias having electronic functionality in addition to thermal conductionmay be filled with a monolithic metal body according to the presentdisclosure as well. These operations may be facilitated since theprocessing conditions used for consolidating metal nanoparticles arefully compatible with the temperature and pressure conditions typicallyemployed for laminating multi-layer PCBs. This approach further allowsthe ready connection of different electrical and thermal planes withinthe board for facilitating heat dissipation throughout a PCB. Directmetallurgical bonding of the monolithic metal body to promote anelectrical connection may be realized by placing the metal nanoparticlecomposition in contact with an electrical interconnect prior to fusingthe metal nanoparticles together with one another.

PCB manufacturing occurs under harsh conditions, oftentimes requiringhigh pressures and temperatures. Each layer may be processed separately,thereby causing distortions, misalignments and thermal stresses. Themore process integration is made possible, the higher the reliabilityand robustness of the final product. The disclosure herein provides forthis high level of process integration in an unparalleled way, with theconformal nature of the metal nanoparticle composition allowing it to beintegrated into production and directly applied to form buried vias,traces and trenches that may accommodate distortions and misalignmentswithout breaking a connection.

As used herein, the term “metal nanoparticle” refers to metal particlesthat are about 100 nm or less in size, without particular reference tothe shape of the metal particles.

As used herein, the term “micron-scale metal particles” refers to metalparticles that are about 100 nm or greater in size in at least onedimension.

The terms “consolidate,” “consolidation” and other variants thereof areused interchangeably herein with the terms “fuse,” “fusion” and othervariants thereof.

As used herein, the terms “partially fused,” “partial fusion,” and otherderivatives and grammatical equivalents thereof refer to the partialcoalescence of metal nanoparticles with one another. Whereas totallyfused metal nanoparticles retain essentially none of the structuralmorphology of the original unfused metal nanoparticles (i.e., theyresemble bulk metal with minimal grain boundaries), partially fusedmetal nanoparticles retain at least some of the structural morphology ofthe original unfused metal nanoparticles and define a grain boundary.The properties of partially fused metal nanoparticles can beintermediate between those of the corresponding bulk metal and theoriginal unfused metal nanoparticles. In some embodiments, fully dense(non-porous) bulk metal can be obtained as a monolithic metal bodywithin a via by the processes described herein. In other embodiments,the bulk metal within a via can have less than about 10% porosity, orless than about 20% porosity, or less than about 30% porosity in anamount above full densification (i.e., >0% porosity). Thus, inparticular embodiments, the bulk metal constituting the monolithic metalbody may have a porosity ranging from about 2% to about 30%, or about 2%to about 5%, or about about 5% to about 10%, or about 10% to about 15%,or about 15% to about 20%, or about 20% to about 25%, or about 25% toabout 30%. In a particular example, a monolithic metal body having auniform porosity of 12% exhibited a thermal conductivity of 277 W/m·K.FIG. 6 shows an SEM image that illustrates porosity in a consolidatedcopper nanoparticle paste composition.

Before further discussing more particular aspects of the presentdisclosure in more detail, additional brief description of metalnanoparticles and their processing conditions, particularly coppernanoparticles, will first be provided. Metal nanoparticles exhibit anumber of properties that can differ significantly from those of thecorresponding bulk metal. One property of metal nanoparticles that canbe of particular importance for processing according to the disclosureherein is nanoparticle fusion (consolidation) that occurs at the metalnanoparticles' fusion temperature. As used herein, the term “fusiontemperature” refers to the temperature at which a metal nanoparticleliquefies, thereby giving the appearance of melting. As used herein, theterms “fusion” and “consolidation” synonymously refer to the coalescenceor partial coalescence of metal nanoparticles with one another to form alarger mass, such as a monolithic metal body filling and extendingthrough a via. Accordingly, there is at least partial connectivitybetween the metal nanoparticles following heating above the fusiontemperature. Following consolidation of the metal nanoparticles, theresulting nanoporosity accommodates thermal stresses occurring duringheat-up and cool-down, as shown in FIG. 5, in comparison to othermaterials commonly employed in PCB manufacturing. As shown, consolidatedcopper nanoparticles exhibit superior stress-strain behavior compared toother materials commonly used in PCB manufacturing. The resistance tothermomechanical stresses compared to conventional PCB materials, asshown in FIG. 5, is believed to arise from uniform nanoporosity in the2-30% range, as discussed above. Moreover, nanoporosity in this rangemay afford ductility and deformation during extreme thermal cyclingoperations, thereby decreasing a risk of failure.

Upon decreasing in size, particularly below about 20 nm in equivalentspherical diameter, the temperature at which metal nanoparticles undergoliquefication drops dramatically from that of the corresponding bulkmetal. For example, copper nanoparticles having a size of about 20 nm orless can have fusion temperatures of about 220° C. or below, or about200° C. or below, in comparison to bulk copper's melting point of 1083°C. Thus, the consolidation of metal nanoparticles taking place at thefusion temperature can allow structures containing bulk metal to befabricated at significantly lower processing temperatures than whenworking directly with the bulk metal itself as a starting material.Processing conditions for consolidating metal nanoparticles aretypically within normal PCB manufacturing parameters of around 375° F.and 275-400 psi. In the case of copper nanoparticles, for example, thefusion temperature is below the temperatures at which commonly used PCBsubstrates undergo melting or distortion. Thus, metal nanoparticles,such as copper nanoparticles, provide a facile material for filling viasin PCBs and forming a monolithic block comprising bulk metal or bulkmetal composite without distorting the board or damaging othercomponents on the board. Accordingly, the use of metal nanoparticlesaccording to the disclosure herein does not require development ofalternative fabrication lines or materials differing from thoseconventionally used in PCB manufacturing.

A number of scalable processes for producing bulk quantities of metalnanoparticles in a targeted size range have been developed. Mosttypically, such processes for producing metal nanoparticles take placeby reducing a metal precursor in the presence of one or moresurfactants. The metal nanoparticles can then be isolated and purifiedfrom the reaction mixture by common isolation techniques and processedinto a formulation suitable for dispensation into vias.

Any suitable technique can be employed for forming the metalnanoparticles used in the disclosure herein. Particularly facile metalnanoparticle fabrication techniques are described in U.S. Pat. Nos.7,736,414, 8,105,414, 8,192,866, 8,486,305, 8,834,747, 9,005,483,9,095,898, and 9,700,940, each of which is incorporated herein byreference in its entirety. As described therein, metal nanoparticles canbe fabricated in a narrow size range by reduction of a metal salt in asolvent in the presence of a suitable surfactant system, which caninclude one or more different surfactants. Further description ofsuitable surfactant systems follows below. Without being bound by anytheory or mechanism, it is believed that the surfactant system canmediate the nucleation and growth of the metal nanoparticles, limitsurface oxidation of the metal nanoparticles, and/or inhibit metalnanoparticles from extensively aggregating with one another prior tobeing at least partially fused together. Suitable organic solvents forsolubilizing metal salts and forming metal nanoparticles can include,for example, formamide, N,N-dimethylformamide, dimethyl sulfoxide,dimethylpropylene urea, hexamethylphosphoramide, tetrahydrofuran, glyme,diglyme, triglyme, tetraglyme, proglyme, or polyglyme. Reducing agentssuitable for reducing metal salts and promoting the formation of metalnanoparticles can include, for example, an alkali metal in the presenceof a suitable catalyst (e.g., lithium naphthalide, sodium naphthalide,or potassium naphthalide) or borohydride reducing agents (e.g., sodiumborohydride, lithium borohydride, potassium borohydride, ortetraalkylammonium borohydrides).

FIGS. 1 and 2 show diagrams of presumed structures of metalnanoparticles having a surfactant coating thereon. As shown in FIG. 1,metal nanoparticle 10 includes metallic core 12 and surfactant layer 14overcoating metallic core 12. Surfactant layer 14 can contain anycombination of surfactants, as described in more detail below. Metalnanoparticle 20, shown in FIG. 2, is similar to that depicted in FIG. 1,except metallic core 12 is grown about nucleus 21, which can be a metalthat is the same as or different than that of metallic core 12. Becausenucleus 21 is buried deep within metallic core 12 in metal nanoparticle20 and is very small in size, it is not believed to significantly affectthe overall nanoparticle properties. Nucleus 21 may comprise a salt or ametal, wherein the metal may be the same or different than metallic core12. In some embodiments, the nanoparticles can have an amorphousmorphology.

As discussed above, the metal nanoparticles have a surfactant coatingcontaining one or more surfactants upon their surface. The surfactantcoating can be formed on the metal nanoparticles during their synthesis.The surfactant coating is generally lost during consolidation of themetal nanoparticles upon heating above the fusion temperature, whichresults in formation of a monolithic metal body within the one or morevias, according to the embodiments of the present disclosure. Formationof a surfactant coating upon metal nanoparticles during their synthesescan desirably limit the ability of the metal nanoparticles to fuse toone another prematurely, limit agglomeration of the metal nanoparticles,and promote the formation of a population of metal nanoparticles havinga narrow size distribution. Porosity values may range from 2-30%, whichmay be tailored based upon a number of factors, including the type ofsurfactant(s) that are present.

The types of metal nanoparticles suitable for use in conjunction withthe various embodiments of the present disclosure are not believed to beparticularly limited. Suitable metal nanoparticles can include, but arenot limited to, tin nanoparticles, copper nanoparticles, aluminumnanoparticles, palladium nanoparticles, silver nanoparticles, goldnanoparticles, iron nanoparticles, cobalt nanoparticles, nickelnanoparticles, titanium nanoparticles, zirconium nanoparticles, hafniumnanoparticles, tantalum nanoparticles, and the like. Micron-sizedparticles of these metals can be present in metal nanoparticle pastecompositions containing the metal nanoparticles as well. Copper can be aparticularly desirable metal for use in the embodiments of the presentdisclosure due to its low cost, strength, and excellent electrical andthermal conductivity values.

In various embodiments, the surfactant system present within the metalnanoparticles can include one or more surfactants. The differingproperties of various surfactants can be used to tailor the propertiesof the metal nanoparticles. Factors that can be taken into account whenselecting a surfactant or combination of surfactants for inclusion uponthe metal nanoparticles can include, for example, ease of surfactantdissipation from the metal nanoparticles during nanoparticle fusion,nucleation and growth rates of the metal nanoparticles, the metalcomponent of the metal nanoparticles, and the like.

In some embodiments, an amine surfactant or combination of aminesurfactants, particularly aliphatic amines, can be present upon themetal nanoparticles. Amine surfactants can be particularly desirable foruse in conjunction with copper nanoparticles. In some embodiments, twoamine surfactants can be used in combination with one another. In otherembodiments, three amine surfactants can be used in combination with oneanother. In more specific embodiments, a primary amine, a secondaryamine, and a diamine chelating agent can be used in combination with oneanother. In still more specific embodiments, the three amine surfactantscan include a long chain primary amine, a secondary amine, and a diaminehaving at least one tertiary alkyl group nitrogen substituent. Furtherdisclosure regarding suitable amine surfactants follows hereinafter.

In some embodiments, the surfactant system can include a primaryalkylamine. In some embodiments, the primary alkylamine can be a C₂-C₁₈alkylamine. In some embodiments, the primary alkylamine can be a C₇-C₁₀alkylamine. In other embodiments, a C₅-C₆ primary alkylamine can also beused. Without being bound by any theory or mechanism, the exact size ofthe primary alkylamine can be balanced between being long enough toprovide an effective inverse micelle structure during synthesis versushaving ready volatility and/or ease of handling during nanoparticleconsolidation. For example, primary alkylamines with more than 18carbons can also be suitable for use in the present embodiments, butthey can be more difficult to handle because of their waxy character.C₇-C₁₀ primary alkylamines, in particular, can represent a good balanceof desired properties for ease of use.

In some embodiments, the C₂-C₁₈ primary alkylamine can be n-hexylamine,n-heptylamine, n-octylamine, n-nonylamine, or n-decylamine, for example.While these are all straight chain primary alkylamines, branched chainprimary alkylamines can also be used in other embodiments. For example,branched chain primary alkylamines such as, for example,7-methyloctylamine, 2-methyloctylamine, or 7-methylnonylamine can beused. In some embodiments, such branched chain primary alkylamines canbe sterically hindered where they are attached to the amine nitrogenatom. Non-limiting examples of such sterically hindered primaryalkylamines can include, for example, t-octylamine,2-methylpentan-2-amine, 2-methylhexan-2-amine, 2-methylheptan-2-amine,3-ethyloctan-3-amine, 3-ethylheptan-3-amine, 3-ethylhexan-3-amine, andthe like. Additional branching can also be present. Without being boundby any theory or mechanism, it is believed that primary alkylamines canserve as ligands in the metal coordination sphere but be readilydissociable therefrom during metal nanoparticle consolidation.

In some embodiments, the surfactant system can include a secondaryamine. Secondary amines suitable for forming metal nanoparticles caninclude normal, branched, or cyclic C₄-C₁₂ alkyl groups bound to theamine nitrogen atom. In some embodiments, the branching can occur on acarbon atom bound to the amine nitrogen atom, thereby producingsignificant steric encumbrance at the nitrogen atom. Suitable secondaryamines can include, without limitation, dihexylamine, diisobutylamine,di-t-butylamine, dineopentylamine, di-t-pentylamine, dicyclopentylamine,dicyclohexylamine, and the like. Secondary amines outside the C₄-C₁₂range can also be used, but such secondary amines can have undesirablephysical properties such as low boiling points or waxy consistenciesthat can complicate their handling.

In some embodiments, the surfactant system can include a chelatingagent, particularly a diamine chelating agent. In some embodiments, oneor both of the nitrogen atoms of the diamine chelating agent can besubstituted with one or two alkyl groups. When two alkyl groups arepresent on the same nitrogen atom, they can be the same or different.Further, when both nitrogen atoms are substituted, the same or differentalkyl groups can be present. In some embodiments, the alkyl groups canbe C₁-C₆ alkyl groups. In other embodiments, the alkyl groups can beC₁-C₄ alkyl groups or C₃-C₆ alkyl groups. In some embodiments, C₃ orhigher alkyl groups can be straight or have branched chains. In someembodiments, C₃ or higher alkyl groups can be cyclic. Without beingbound by any theory or mechanism, it is believed that diamine chelatingagents can facilitate metal nanoparticle formation by promotingnanoparticle nucleation.

In some embodiments, suitable diamine chelating agents can includeN,N′-dialkylethylenediamines, particularly C₁-C₄N,N′-dialkylethylenediamines. The corresponding methylenediamine,propylenediamine, butylenediamine, pentylenediamine or hexylenediaminederivatives can also be used. The alkyl groups can be the same ordifferent. C₁-C₄ alkyl groups that can be present include, for example,methyl, ethyl, propyl, and butyl groups, or branched alkyl groups suchas isopropyl, isobutyl, s-butyl, and t-butyl groups. IllustrativeN,N′-dialkylethylenediamines that can be suitable for inclusion uponmetal nanoparticles include, for example,N,N′-di-t-butylethylenediamine, N,N′-diisopropylethylenediamine, and thelike.

In some embodiments, suitable diamine chelating agents can includeN,N,N′,N′-tetraalkylethylenediamines, particularly C₁-C₄N,N,N′,N′-tetraalkylethylenediamines. The correspondingmethylenediamine, propylenediamine, butylenediamine, pentylenediamine orhexylenediamine derivatives can also be used. The alkyl groups can againbe the same or different and include those mentioned above. IllustrativeN,N,N′,N′-tetraalkylethylenediamines that can be suitable for use informing metal nanoparticles include, for example,N,N,N′,N′-tetramethylethylenediamine,N,N,N′,N′-tetraethylethylenediamine, and the like.

Surfactants other than aliphatic amines can also be present in thesurfactant system. In this regard, suitable surfactants can include, forexample, pyridines, aromatic amines, phosphines, thiols, or anycombination thereof. These surfactants can be used in combination withan aliphatic amine, including those described above, or they can be usedin a surfactant system in which an aliphatic amine is not present.Further disclosure regarding suitable pyridines, aromatic amines,phosphines, and thiols follows below.

Suitable aromatic amines can have a formula of ArNR¹R², where Ar is asubstituted or unsubstituted aryl group and R¹ and R² are the same ordifferent. R¹ and R² can be independently selected from H or an alkyl oraryl group containing from 1 to about 16 carbon atoms. Illustrativearomatic amines that can be suitable for use in forming metalnanoparticles include, for example, aniline, toluidine, anisidine,N,N-dimethylaniline, N,N-diethylaniline, and the like. Other aromaticamines that can be used in conjunction with metal nanoparticles can beenvisioned by one having ordinary skill in the art.

Suitable pyridines can include both pyridine and its derivatives.Illustrative pyridines that can be suitable for use inclusion upon metalnanoparticles include, for example, pyridine, 2-methylpyridine,2,6-dimethylpyridine, collidine, pyridazine, and the like. Chelatingpyridines such as bipyridyl chelating agents may also be used. Otherpyridines that can be used in conjunction with metal nanoparticles canbe envisioned by one having ordinary skill in the art.

Suitable phosphines can have a formula of PR₃, where R is an alkyl oraryl group containing from 1 to about 16 carbon atoms. The alkyl or arylgroups attached to the phosphorus center can be the same or different.Illustrative phosphines that can be present upon metal nanoparticlesinclude, for example, trimethylphosphine, triethylphosphine,tributylphosphine, tri-t-butylphosphine, trioctylphosphine,triphenylphosphine, and the like. Phosphine oxides can also be used in alike manner. In some embodiments, surfactants that contain two or morephosphine groups configured for forming a chelate ring can also be used.Illustrative chelating phosphines can include 1,2-bisphosphines,1,3-bisphosphines, and bis-phosphines such as BINAP, for example. Otherphosphines that can be used in conjunction with metal nanoparticles canbe envisioned by one having ordinary skill in the art.

Suitable thiols can have a formula of RSH, where R is an alkyl or arylgroup having from about 4 to about 16 carbon atoms. Illustrative thiolsthat can present upon metal nanoparticles include, for example,butanethiol, 2-methyl-2-propanethiol, hexanethiol, octanethiol,benzenethiol, and the like. In some embodiments, surfactants thatcontain two or more thiol groups configured for forming a chelate ringcan also be used. Illustrative chelating thiols can include, forexample, 1,2-dithiols (e.g., 1,2-ethanethiol) and 1,3-dithiols (e.g.,1,3-propanethiol). Other thiols that can be used in conjunction withmetal nanoparticles can be envisioned by one having ordinary skill inthe art.

As mentioned above, a distinguishing feature of metal nanoparticles istheir low fusion temperature, which may facilitate via filling toestablish an effective thermal conduction pathway therein, according tothe various embodiments of the present disclosure. In order tofacilitate their disposition and formation of a monolithic metal bodywithin the vias, the metal nanoparticles may be incorporated in a pasteor similar formulation suitable for controlled introduction into thevias. Additional disclosure directed to metal nanoparticle pastecompositions and similar formulations follows hereinbelow.

Metal nanoparticle paste compositions or similar formulations can beprepared by dispersing as-produced or as-isolated metal nanoparticles inan organic matrix containing one or more organic solvents and variousother optional components. As used herein, the terms “nanoparticle pasteformulation,” “nanoparticle paste composition” and grammaticalequivalents thereof are used interchangeably and refer synonymously to afluid composition containing dispersed metal nanoparticles that issuitable for dispensation using a desired technique. Use of the term“paste” does not necessarily imply an adhesive function of the pastealone. Through judicious choice of the organic solvent(s) and otheradditives, the loading of metal nanoparticles and the like, readydispensation of the metal nanoparticles within one or more vias may bepromoted.

Cracking can sometimes occur during consolidation of the metalnanoparticles. One way in which the nanoparticle pastes of the presentdisclosure can promote a decreased degree of cracking and void formationfollowing metal nanoparticle consolidation is by maintaining a highsolids content. More particularly, in some embodiments, the pastecompositions can contain at least about 30% metal nanoparticles byweight, particularly about 30% to about 98% metal nanoparticles byweight of the paste composition, or about 50% to about 95% metalnanoparticles by weight of the paste composition, or about 70% to about98% metal nanoparticles by weight of the paste composition. Moreover, insome embodiments, small amounts (e.g., about 0.01% to about 15% or about35% or about 60% by weight of the paste composition) of micron-scalemetal particles can be present in addition to the metal nanoparticles.Such micron-scale metal particles can desirably promote the fusion ofmetal nanoparticles into a contiguous mass (monolithic metal body) andfurther reduce the incidence of cracking. Instead of being liquefied andundergoing direct consolidation as is the case for the metalnanoparticles, the micron-scale metal particles can simply become joinedtogether upon being contacted with liquefied metal nanoparticles thathave been raised above their fusion temperature. These factors canreduce the porosity that results after fusing the metal nanoparticlestogether. The micron-scale metal particles can contain the same ordifferent metals than the metal nanoparticles, and suitable metals forthe micron-scale particles can include, for example, copper, silver,gold, aluminum, tin, and the like. Micron-scale graphite particles mayalso be included, in some embodiments. Carbon nanotubes, diamondparticles, and/or graphene may be included, in some embodiments.Carbonaceous additives may increase the thermal conductivity resultingafter metal nanoparticle consolidation takes place, according to someembodiments.

Decreased cracking and void formation during metal nanoparticleconsolidation can also be promoted by judicious choice of the solvent(s)forming the organic matrix. A tailored combination of organic solventscan desirably decrease the incidence of cracking and void formation.More particularly, an organic matrix containing one or more hydrocarbons(saturated, monounsaturated, polyunsaturated (2 or more double bonds) oraromatic), one or more alcohols, one or more amines, and one or moreorganic acids can be especially effective for this purpose. One or moreesters and/or one or more anhydrides may be included, in someembodiments. Without being bound by any theory or mechanism, it isbelieved that this combination of organic solvents can facilitate theremoval and sequestration of surfactant molecules surrounding the metalnanoparticles during consolidation, such that the metal nanoparticlescan more easily fuse together with one another. More particularly, it isbelieved that hydrocarbon and alcohol solvents can passively solubilizesurfactant molecules released from the metal nanoparticles by Brownianmotion and reduce their ability to become re-attached thereto. Inconcert with the passive solubilization of surfactant molecules, amineand organic acid solvents can actively sequester the surfactantmolecules through a chemical interaction such that they are no longeravailable for recombination with the metal nanoparticles.

Further tailoring of the solvent composition can be performed to reducethe suddenness of volume contraction that takes place during surfactantremoval and metal nanoparticle consolidation. Specifically, more thanone member of each class of organic solvent (i.e., hydrocarbons,alcohols, amines, and organic acids), can be present in the organicmatrix, where the members of each class have boiling points that areseparated from one another by a set degree. For example, in someembodiments, the various members of each class can have boiling pointsthat are separated from one another by about 20° C. to about 50° C. Byusing such a solvent mixture, sudden volume changes due to rapid loss ofsolvent can be minimized during metal nanoparticle consolidation, sincethe various components of the solvent mixture can be removed graduallyover a broad range of boiling points (e.g., about 50° C. to about 200°C.).

In various embodiments, at least some of the one or more organicsolvents can have a boiling point of about 100° C. or greater. In othervarious embodiments, at least some of the one or more organic solventscan have a boiling point of about 200° C. or greater. In some or otherembodiments, the one or more organic solvents can have boiling pointsranging between about 50° C. and about 200° C., or between about 50° C.and about 250° C., or between about 50° C. and about 300° C., or betweenabout 50° C. and about 350° C. Use of high boiling organic solvents candesirably increase the pot life of the metal nanoparticle pastecompositions and limit the rapid loss of solvent, which can lead tocracking and void formation during nanoparticle consolidation. In someembodiments, at least one of the organic solvents can have a boilingpoint that is higher than the boiling point(s) of the surfactant(s)associated with the metal nanoparticles. Accordingly, surfactant(s) canbe removed from the metal nanoparticles by evaporation before removal ofthe organic solvent(s) takes place.

In some embodiments, the organic matrix can contain one or morealcohols, which may be C₂-C₁₂, C₄-C₁₂ or C₇-C₁₂ in more particularembodiments. In various embodiments, the alcohols can include monohydricalcohols, diols, or triols. One or more glycol ethers (e.g., diethyleneglycol and triethylene glycol), alkanolamines (e.g., ethanolamine,triethanolamine, and the like), or any combination thereof may bepresent in certain embodiments, which may be present alone or incombination with other alcohols. Various glymes may be present with theone or more alcohols in some embodiments. In some embodiments, one ormore hydrocarbons can be present in combination with one or morealcohols. As discussed above, it is believed that alcohol (andoptionally glymes and alkanolamines) and hydrocarbon solvents canpassively promote the solubilization of surfactants as they are removedfrom the metal nanoparticles by Brownian motion and limit theirre-association with the metal nanoparticles. Moreover, hydrocarbon andalcohol solvents only weakly coordinate with metal nanoparticles, sothey do not simply replace the displaced surfactants in the nanoparticlecoordination sphere. Illustrative but non-limiting examples of alcoholand hydrocarbon solvents that can be present include, for example, lightaromatic petroleum distillate (CAS 64742-95-6), hydrotreated lightpetroleum distillates (CAS 64742-47-8), tripropyleneglycol methyl ether,ligroin (CAS 68551-17-7, a mixture of C₁₀-C₁₃ alkanes),diisopropyleneglycol monomethyl ether, diethyleneglycol diethyl ether,2-propanol, 2-butanol, t-butanol, 1-hexanol, 2-(2-butoxyethoxy)ethanol,and terpineol. In some embodiments, polyketone solvents can be used in alike manner.

In some embodiments, the organic matrix can contain one or more aminesand one or more organic acids. In some embodiments, the one or moreamines and one or more organic acids can be present in an organic matrixthat also includes one or more hydrocarbons and one or more alcohols. Asdiscussed above, it is believed that amines and organic acids canactively sequester surfactants that have been passively solubilized byhydrocarbon and alcohol solvents, thereby making the surfactantsunavailable for re-association with the metal nanoparticles. Thus, anorganic solvent that contains a combination of one or more hydrocarbons,one or more alcohols, one or more amines, and one or more organic acidscan provide synergistic benefits for promoting the consolidation ofmetal nanoparticles. Illustrative but non-limiting examples of aminesolvents that can be present include, for example, tallowamine (CAS61790-33-8), alkyl (C₈-C₁₈) unsaturated amines (CAS 68037-94-5),di(hydrogenated tallow)amine (CAS 61789-79-5), dialkyl (C₈-C₂₀) amines(CAS 68526-63-6), alkyl (C₁₀-C₁₆)dimethyl amine (CAS 67700-98-5), alkyl(C₁₄-C₁₈) dimethyl amine (CAS 68037-93-4), dihydrogenated tallowmethylamine (CAS 61788-63-4), and trialkyl (C₆-C₁₂) amines (CAS 68038-01-7).Illustrative but non-limiting examples of organic acid solvents that canbe present in the nanoparticle paste compositions include, for example,octanoic acid, nonanoic acid, decanoic acid, caprylic acid, pelargonicacid, undecylic acid, lauric acid, tridecylic acid, myristic acid,pentadecanoic acid, palmitic acid, margaric acid, stearic acid,nonadecylic acid, α-linolenic acid, stearidonic acid, oleic acid, andlinoleic acid.

In some embodiments, the organic matrix can include more than onehydrocarbon, more than one alcohol, optionally more than one glyme(glycol ether), more than one amine, and more than one organic acid. Forexample, in some embodiments, each class of organic solvent can have twoor more members, or three or more members, or four or more members, orfive or more members, or six or more members, or seven or more members,or eight or more members, or nine or more members, or ten or moremembers. Moreover, the number of members in each class of organicsolvent can be the same or different. Particular benefits of usingmultiple members of each class of organic solvent are describedhereinafter.

One particular advantage of using multiple members within each class oforganic solvent can include the ability to provide a wide spread ofboiling points in the metal nanoparticle paste compositions. Byproviding a wide spread of boiling points, the organic solvents can beremoved gradually as the temperature rises while affecting metalnanoparticle consolidation, thereby limiting volume contraction anddisfavoring cracking. By gradually removing the organic solvent in thismanner, less temperature control may be needed to affect slow solventremoval than if a single solvent with a narrow boiling point range wasused. In some embodiments, the members within each class of organicsolvent can have a window of boiling points ranging between about 50° C.and about 200° C., or between about 50° C. and about 250° C., or betweenabout 100° C. and about 200° C., or between about 100° C. and about 250°C., or between about 150° C. and about 300° C., or between about 150° C.and about 350° C. In more particular embodiments, the various members ofeach class of organic solvent can each have boiling points that areseparated from one another by at least about 20° C., specifically about20° C. to about 50° C. More specifically, in some embodiments, eachhydrocarbon can have a boiling point that differs by about 20° C. toabout 50° C. from other hydrocarbons in the organic matrix, each alcoholcan have a boiling point that differs by about 20° C. to about 50° C.from other alcohols in the organic matrix, each amine can have a boilingpoint that differs by about 20° C. to about 50° C. from other amines inthe organic matrix, and each organic acid can have a boiling point thatdiffers by about 20° C. to about 50° C. from other organic acids in theorganic matrix. The more members of each class of organic solvent thatare present, the smaller the differences become between the boilingpoints. By having smaller differences between the boiling points,solvent removal can be made more continual, thereby limiting the degreeof volume contraction that occurs at each stage. A reduced degree ofcracking can occur when four to five or more members of each class oforganic solvent are present (e.g., four or more hydrocarbons, four ormore alcohols, four or more amines, and four or more organic acids; orfive or more hydrocarbons, five or more alcohols, five or more amines,and five or more organic acids), each having boiling points that areseparated from one another within the above range.

In various embodiments, the metal nanoparticles used in the metalnanoparticle paste compositions can be about 20 nm or less in size. Inother various embodiments, metal nanoparticles may be up to about 75 nmin size. As discussed above, metal nanoparticles in this size range havefusion temperatures that are significantly lower than those of thecorresponding bulk metal and readily undergo consolidation with oneanother as a result. In some embodiments, metal nanoparticles that areabout 20 nm or less in size can have a fusion temperature of about 220°C. or below (e.g., a fusion temperature in the range of about 140° C. toabout 220° C.) or about 200° C. or below, which can provide advantagesthat are noted above. In some embodiments, at least a portion of themetal nanoparticles can be about 10 nm or less in size, or about 5 nm orless in size. In more specific embodiments, at least a portion of themetal nanoparticles can range from about 1 nm in size to about 20 nm insize, or from about 1 nm in size and about 10 nm in size, or from about1 nm in size to about 5 nm in size, or from about 3 nm in size to about7 nm in size, or from about 5 nm in size to about 20 nm in size. In someembodiments, substantially all of the metal nanoparticles can residewithin these size ranges. In some embodiments, larger metalnanoparticles can be combined in the metal nanoparticle pastecompositions with metal nanoparticles that are about 20 nm in size orless. For example, in some embodiments, metal nanoparticles ranging fromabout 1 nm to about 10 nm in size can be combined with metalnanoparticles that range from about 25 nm to about 50 nm in size, orwith metal nanoparticles that range from about 25 nm to about 100 nm insize. As further discussed below, micron-scale metal particles and/ornanoscale particles can also be included in the metal nanoparticle pastecompositions in some embodiments. Although larger metal nanoparticlesand micron-scale metal particles may not be liquefiable the lowtemperatures of their smaller counterparts, they can still becomeconsolidated upon contacting the smaller metal nanoparticles that havebeen liquefied at or above their fusion temperature, as generallydiscussed above.

In addition to metal nanoparticles and organic solvents, other additivescan also be present in the metal nanoparticle paste compositions. Suchadditional additives can include, for example, rheology control aids,thickening agents, micron-scale conductive additives, nanoscaleconductive additives, and any combination thereof. Chemical additivescan also be present. As discussed hereinafter, the inclusion ofmicron-scale conductive additives, such as micron-scale metal particles,can be particularly advantageous. Nanoscale or micron-scale diamond orother thermally conductive additives may be desirable to include in someinstances for promoting more efficient heat transfer.

In some embodiments, the paste compositions can contain about 0.01% toabout 15% micron-scale metal particles by weight, or about 1% to about10% micron-scale metal particles by weight, or about 1% to about 5%micron-scale metal particles by weight, or about 0.1% to about 35%micron-scale metal particles by weight. Inclusion of micron-scale metalparticles in the metal nanoparticle paste compositions can desirablyreduce the incidence of cracking that occurs during consolidation of themetal nanoparticles when forming a monolithic metal body. Without beingbound by any theory or mechanism, it is believed that the micron-scalemetal particles can become consolidated with one another as the metalnanoparticles are liquefied and form a transient liquid coating upon thesurface of the micron-scale metal particles. In some embodiments, themicron-scale metal particles can range from about 500 nm to about 100microns in size in at least one dimension, or from about 500 nm to about10 microns in size in at least one dimension, or from about 100 nm toabout 5 microns in size in at least one dimension, or from about 100 nmto about 10 microns in size in at least one dimension, or from about 100nm to about 1 micron in size in at least one dimension, or from about 1micron to about 10 microns in size in at least one dimension, or fromabout 5 microns to about 10 microns in size in at least one dimension,or from about 1 micron to about 100 microns in size in at least onedimension. The micron-size metal particles can contain the same metal asthe metal nanoparticles or contain a different metal. Thus, metal alloyscan be fabricated by including micron-size metal particles in the pastecompositions with a metal differing from that of the metalnanoparticles. Metal alloys may also be formed by combining differenttypes of metal nanoparticles with one another. Suitable micron-scalemetal particles can include, for example, Cu, Ni, Al, Fe, Co, Mo, Ag,Zn, Sn, Au, Pd, Pt, Ru, Mn, Cr, Ti, V, Mg or Ca particles. Non-metalparticles such as, for example, Si and B micron-scale particles can beused in a like manner. In some embodiments, the micron-scale metalparticles can be in the form of metal flakes, such as high aspect ratiocopper flakes, for example. Thus, in some embodiments, the metalnanoparticle paste compositions described herein can contain a mixtureof copper nanoparticles and high aspect ratio copper flakes or anothertype of micron-scale copper particles. Specifically, in someembodiments, the metal nanoparticle paste compositions can contain about30% to about 90% copper nanoparticles by weight and about 0.01% to about15% or 1% to 35% high aspect ratio copper flakes by weight.

Other micron-scale metal particles that can be used equivalently to highaspect ratio metal flakes include, for example, metal nanowires andother high aspect ratio particles, which can be up to about 300 micronsin length. The ratio of metal nanoparticles to metal nanowires may rangebetween about 10:1 to about 40:1, according to various embodiments.Suitable nanowires may have a length of about 5 microns to about 50microns, and a diameter of about 100 nm to about 200 nm, for example.

In some embodiments, nanoscale conductive additives can also be presentin the metal nanoparticle paste compositions. These additives candesirably provide further structural stabilization and reduce shrinkageduring metal nanoparticle consolidation. Moreover, inclusion ofnanoscale conductive additives can increase electrical and thermalconductivity values that can approach or even exceed that of thecorresponding bulk metal following nanoparticle consolidation, which canbe desirable for promoting heat transfer according to the disclosureherein. In some embodiments, the nanoscale conductive additives can havea size in at least one dimension ranging from about 1 micron to about100 microns or ranging from about 1 micron to about 300 microns.Suitable nanoscale conductive additives can include, for example, carbonnanotubes, graphene, nanodiamond, and the like. When present, the metalnanoparticle paste compositions can contain about 1% to about 10%nanoscale conductive additives by weight, or about 1% to about 5%nanoscale conductive additives by weight.

Additional substances that can also optionally be present in the metalnanoparticle paste compositions include, for example, flame retardants,UV protective agents, antioxidants, carbon black, graphite, fibermaterials (e.g., chopped carbon fiber materials), diamond, and the like.

In some more specific embodiments, suitable nanoparticle pastecompositions may further comprise diamond particles. A suitable size ofdiamond particles may be dictated by the size of the via to be filled.In general, diamond particles smaller in size than the diameter of thevia may be selected so that the diamond particles can successfully enterthe via. Moreover, the diamond particles may be sized as large aspossible, taking into account the diameter of the via, to limit grainboundaries that need to be crossed by phonons during heat transfer. Thesize of the diamond particles may remain sufficiently small such thatdispensability of the metal nanoparticle paste composition is notcompromised.

In still more specific embodiments, diamond particles suitable for usein the metal nanoparticle paste compositions may have a size rangingfrom about 1 micron to about 1000 microns, which can provide for goodparticle dispersion and acceptable paste dispensibility. Diamondparticles having a size ranging from about 200 microns to about 250microns can represent a good compromise between providing effectivedispersion and a minimized grain boundary for discouraging phononscattering. Other suitable size ranges for the diamond particles canrange from about 25 microns to about 150 microns, or about 50 microns toabout 250 microns, or from about 100 microns to about 250 microns, orfrom about 100 microns to about 200 microns, or from about 150 micronsto about 250 microns.

In illustrative embodiments, the vias can include about 10% to about 75%diamond particles by volume after metal nanoparticle consolidation hastaken place to form a monolithic metal body. That is, according to someembodiments of the present disclosure, the vias may be filled with amonolithic metal body that is a metal-diamond composite, wherein themetal-diamond composite extends longitudinally through a via from afirst face to a second face of the PCB.

Admixture of copper nanoparticles and diamond particles may be desirablefor filling vias for several reasons. Copper is low in cost compared tomost other metals, is impedance matched relatively well with diamond,and bears high thermal conductivity on its own. In some embodiments,impedance matching can be further improved by including acarbide-forming additive to form a thin layer (single atom to <10 nmthick layer) of carbide upon the diamond particles. As such, thecombination of copper nanoparticles and diamond particles can providevery effective heat transfer in the various embodiments of the presentdisclosure. In the case where the vias also establish electroniccommunication between various board layers, copper also affords highelectrical conductivity as well.

Nanoparticle paste compositions suitable for use in filling vias orsimilar conduits according to the present disclosure can be formulatedusing any of the metal nanoparticle paste compositions describedhereinabove. In addition, according to some embodiments, multiple metalsmay be present in the metal nanoparticle paste compositions. In some orother embodiments, suitable metal nanoparticle paste compositions caninclude a mixture of metal nanoparticles, other nano-sized particles(i.e., particles having a dimension of about 100 nm or less), and/ormicron-sized particles. The metal nanoparticle paste compositions maycomprise copper nanoparticles, according to more specific embodiments.

In various embodiments of the present disclosure, new or previouslyexisting vias in a circuit board (e.g., a PCB) may be filled with metalnanoparticles, which undergo subsequent consolidation. The metalnanoparticles may be contained within a metal nanoparticle pastecomposition. The metal nanoparticle composition is subsequentlyprocessed to promote at least partial consolidation of the metalnanoparticles within the vias to form a monolithic metal body (acontinuous mass of bulk metal) that fills the vias and extends betweenopposing faces of the circuit board. For example, according to variousembodiments, the metal nanoparticles may be heated at or above thefusion temperature while located within the via to form the monolithicmetal body. Alternately, pressure may be applied to promote metalnanoparticle consolidation within the vias. Once the monolithic metalbody has been formed within the vias, one or both ends of the monolithicmetal body may be connected to a heat sink or like heat dissipationapparatus of various types. The vias and the monolithic metal bodiesextend between the first and second faces of the circuit board, therebyallowing excess heat to be shunted from one side of the circuit board tothe other.

Vias are easily integrated into a PCB and may be drilled out afterinitial assembly. Alternately, vias extending through the PCB may bedefined during lamination of the substrate layers in multi-layer PCBs.The vias can have a range of sizes, of they can all be substantially thesame size. The size range of the vias that may be filled with metalnanoparticles according to the present disclosure is not considered tobe particularly limited and may range from micron-sized up to sizeslarger than one millimeter or more. According to more specificembodiments, the vias can range in size (diameter) from about 100microns up to about 30 mm or 50 mm. In still more specific embodiments,the vias can range from about 1 mm to about 10 mm, or from 1 mm to about8 mm, or from about 1 mm to about 5 mm, or from about 2 mm to about 5mm, or from about 3 mm to about 10 mm, or from about 2 mm to about 15mm, or from about 3 mm to about 15 mm. In some embodiments, at least aportion of the vias may be about 5 mm or less in size. In some or otherembodiments, at least a portion of the vias may be about 2 mm to about10 mm in size, or about 3 mm to about 10 mm in size, or about 4 mm toabout 12 mm in size. The size(s) of the vias may dictate to some degreethe manner in which they are filled with a metal nanoparticle pastecomposition, such as a copper nanoparticle paste composition.

Suitable vias may have any cross-sectional profile as they extendthrough the PCB. According to some embodiments, the vias may have around cross-sectional profile; thus, such vias have a cylindrical shape.Other suitable via cross-sectional profiles include, but are not limitedto, square, rectangular, triangular, ovular, or other regular orirregular geometric shapes. The cross-sectional profile of the vias maybe substantially equal in size upon both faces of the PCB, or thecross-sectional profiles may differ in size, according to someembodiments. That is, the vias may be tapered, in some embodiments. Forexample, in some embodiments, suitable vias may have a conical shape,which may allow greater heat dissipation to take place on one side ofthe PCB. Specifically, a conical-shaped via may function as an internalheat spreader, according to various embodiments.

In still further embodiments, the PCBs described herein may also includevias that extend laterally within the PCB (i.e., extending parallel tothe first and second faces of the PCB substrate), as opposed to thethrough-plane vias described above. The vias extending laterally maylikewise be filled with a metal nanoparticle paste composition, whichmay then be consolidated to a monolithic metal body, which may then beused for promoting lateral heat transfer. As such, PCBs containinglateral vias may be connected laterally to a heat sink in addition tobeing coupled to a heat sink at one or both of the PCB faces (i.e., inthermal contact with the through-plane vias). The heat sink connectedlaterally to the lateral vias of the PCBs may be the same heat sinkconnected to the through-plane vias or a different heat sink, accordingto various embodiments.

The PCBs and vias incorporated therein may be single-layer ormulti-layer, according to various embodiments. Multi-layer PCBs cancontain individual layers that are laminated together to define the viasand other board features. Both single-layer and multi-layer PCBs may beformed initially without the through-plane vias being present, in whichcase the vias may then be drilled after fabrication of the board iscomplete.

Once the vias have been filled and the metal nanoparticles undergo atleast partial consolidation (e.g., by heating to at least the fusiontemperature and/or by applying pressure) to form a monolithic metal bodyin the form of a continuous mass, plug, rod, wire, or similar body thatextends completely from one face of the PCB to the other. The monolithicmetal body within the via may contain additional conductive additives,such as graphene, carbon nanotubes, micron-sized diamond, or the like tofurther aid in promoting thermal conductivity, wherein the conductiveadditives are introduced to the vias with the metal nanoparticles (e.g.,within a metal nanoparticle paste composition). Once formed, one or bothends of the monolithic metal body may be connected to a heat sink topromote thermal transfer from the PCB. In more specific embodiments, thetop face of the PCB can be connected to a first heat sink (active orpassive heat dissipation device), as done for conventional PCBcomponents, and the bottom face of the PCB can be connected to a secondheat sink (active or passive heat dissipation device). Thus, themetal-filled vias produced by the processes described herein may allowat least a portion of the excess heat produced on the front side of thePCB to be conveyed to the backside of the PCB, whereupon it can beremoved by the second heat sink. In more specific embodiments, thesecond heat sink on the second face of the PCB can be another heat sinkwith a fan, a liquid cooling device microchannel device, or anycombination thereof. Since the second heat sink is generally locatednear the chassis and is thereby nearer a larger bonding frame, it can beconnected directly to the casing and further to the outside of a device,thereby allowing for more powerful heat dissipation methods to be used.This type of configuration may be especially effective for server farms.For example, in the case of a computer casing housing a circuit board,the computer casing may function as a large-area passive heatdissipation device to facilitate thermal management. Heat pipes sizedfor use with a PCB may also be suitable for use in some embodiments.Provided that vibrations are effectively dampened, heat straps may alsobe used.

Alternately, one of the heat sinks may be placed in proximity to thefirst face of the second face and the other heat sink may be in contactwith the first face and the second face. Thus, one of the heat sinks maybe in contact with an electronic component of the PCB producing excessheat, and the other of the heat sinks may be located on the oppositeface of the PCB, thereby allowing dissipation of the excess heat to takeplace from both faces of the PCB. The two heat sinks may dissipateexcess heat by the same mechanism or by different mechanisms.

Thus, according to various embodiments of the present disclosure, a heatsink may be thermally coupled to one or both faces of a PCB, with eachheat sink being in contact with a face of the PCB substrate or inproximity to a face of the PCB substrate. At least one of the heat sinkscontacts the monolithic metal bodies defined within the via. Whenmultiple heat sinks are present, one heat sink can be active and theother passive, both heat sinks can be active, or both heat sinks can bepassive. The heat sink(s) may directly contact the monolithic metalbodies in particular embodiments of the present disclosure. In some orother particular embodiments, the heat sink(s) may feature a directmetallurgical bond to one or both faces of the monolithic metal bodies.Direct metallurgical bonds may be produced by contacting the heat sinkwith the metal nanoparticle paste composition prior to consolidation toform the monolithic metal bodies.

Therefore, according to various embodiments of the present disclosure,printed circuit boards described herein may comprise a substrate havingone or more vias extending therethrough between a first face and asecond face, a monolithic metal body formed from metal nanoparticlesfilling each of the one or more vias, and heat sink upon at least one ofthe first face or the second face, such that the heat sink directlycontacts the monolithic metal bodies upon at least one face of the PCB.In particular embodiments, the porosity of the monolithic metal body inthe PCB may range from about 2% to about 30% or any subrange thereof, asdiscussed in more detail above.

The heat sink may be directly built on top of the via surface usingadditive manufacturing using the same or a different metal nanoparticlecomposition. This approach may afford direct intimate thermal contactwithout an additional thermal interface to reduce heat transferefficiency. In more particular embodiments, as discussed above, the heatsink may feature a direct metallurgical bond to the monolithic metalbody.

In more particular embodiments, a first heat sink may be located upon oradjacent to the first face and a second heat sink may be located upon oradjacent to the second face. One of the first heat sink or the secondheat sink may contact a heat-producing electronic component, forexample, and shunt excess heat from the PCB without that portion of theexcess heat passing through the PCB substrate. The other of the firstheat sink and the second heat sink may dissipate excess heat conveyed tothe opposite face of the PCB substrate. The first heat sink and thesecond heat sink may dissipate heat by different mechanisms, accordingto some embodiments.

In more particular embodiments, the monolithic metal body may comprisecopper and be formed from copper nanoparticles.

In some or other more particular embodiments, the monolithic metal bodymay further comprise a plurality of diamond particles or other types ofhighly conductive particles (e.g., carbon nanotubes, graphene, or thelike).

In some embodiments, at least a portion of the one or more vias maycontain a reinforcement material. As discussed above, the reinforcementmaterial may aid in retaining the metal nanoparticles within the one ormore vias prior to consolidation of the metal nanoparticles takingplace. Suitable reinforcement materials are considered in more detailbelow. The reinforcement material may extend through all or part of themonolithic metal body once metal nanoparticle consolidation has takenplace.

Methods of the present disclosure may comprise providing a printedcircuit board (PCB) defined upon a substrate and having one or more viasextending through the substrate between a first face and a second face,filling the one or more vias with a metal nanoparticle pastecomposition, consolidating metal nanoparticles of the metal nanoparticlepaste composition within the one or more vias to form a monolithic metalbody filling each of the one or more vias, and placing a heat sink uponat least of the first face or the second face, such that the heat sinkcontacts each monolithic metal body. The metal nanoparticles maycomprise copper nanoparticles in more specific embodiments of thepresent disclosure. The metal nanoparticle paste composition may beadapated to limit shrinkage during consolidation in particularembodiments of the present disclosure, as referenced above.

Suitable metal nanoparticle paste compositions are described in moredetail above. In more particular embodiments, the metal nanoparticlepaste compositions used for filling the one or more vias may comprisemetal nanoparticles (e.g., copper nanoparticles) and at least oneadditive selected from the group consisting of diamond particles,micron-scale metal particles, and any combination thereof. As discussedabove, it may be particularly advantageous for diamond particles,including micron-scale diamond particles and/or nano-scale diamondparticles, to be present in the metal nanoparticle paste compositions,thereby leading to formation of a metal-diamond composite followingnanoparticle consolidation.

Filling of the vias with metal nanoparticles may take place by anysuitable technique. Suitable techniques may include those in which theone or more vias are filled individually with metal nanoparticles, orthose in which the one or more vias are filled substantially at the sametime. In illustrative embodiments, syringes, syringe arrays, or otherarrangements of dispensation devices containing the metal nanoparticlepaste compositions may be used for filling the one or more vias. Inother illustrative embodiments, printing techniques such as screenprinting, stencil printing, or inkjet printing may be used to fill theone or more vias with the metal nanoparticle paste composition. In morespecific embodiments, at least a portion of the one or more vias may befilled by screen printing the metal nanoparticle paste composition.

The technique for filling the vias may vary depending upon the size(diameter) of the vias. In illustrative embodiments, smaller vias, suchas those about 2 mm or about 3 mm in size or below, can be filledindividually using a syringe or similar dispensation device. Automateddispensation into each via may take place in some embodiments, such aswith a robotically operated syringe, optionally incorporating a syringepump. Arrays of syringes or similar arrangements of dispensation devicesmay be used to fill multiple vias simultaneously or near simultaneously.Larger vias, such as those that are about 2 mm to about 7 mm in size or3 mm to about 7 mm in size, or even larger, can also be filledindividually or simultaneously using a syringe or similar dispensationdevice, in some embodiments, or more typically, by stencil printing orscreen printing techniques using a suitable print head. As such, thevias may be filled one at a time in some embodiments, or multiple viasmay be filled with the metal nanoparticle paste compositions at the sametime, according to other embodiments. In some embodiments, each of thevias may be filled with the nanoparticle paste composition at the sametime.

In some embodiments, vias of sufficiently large size may include areinforcement material aid in retaining the metal nanoparticle pastecomposition in place while filling the vias and consolidating the metalnanoparticles. Suitable reinforcement materials may include, forexample, aluminum or copper wire mesh, foam, felt or wool. In variousembodiments, the mesh, foam, felt or wool may be placed in the viasprior to dispensing the metal nanoparticle paste composition therein.The reinforcement material may interpenetrate within the monolithicmetal body within the one or more vias following metal nanoparticleconsolidation. Accordingly, in some embodiments, the methods of thepresent disclosure may comprise placing a reinforcement material in atleast a portion of the one or more vias prior to filling the one or morevias with the metal nanoparticle paste composition.

In addition or alternately, in some embodiments, the metal nanoparticlepaste composition may have a density that is adjusted to minimizesagging, thereby aiding in retaining the nanoparticle paste compositionwithin the one or more vias. In illustrative embodiments, the metalnanoparticle paste compositions may suitably have a density ranging fromabout 3.5 g/cm³ to about 6 g/cm³. Higher density values may be moredesirable to minimize shrinking and cracking during metal nanoparticleconsolidation. Micron-size particles, typically in an amount of about 10wt. % or more, up to about 35 wt. %, may aid in mitigating shrinkageduring metal nanoparticle consolidation.

In some or other embodiments, a removable support may be providedagainst one side of the vias (e.g., upon the face of the PCB oppositethe face where the metal nanoparticle paste composition is loaded intothe one or more vias) to preclude sagging and leakage of thenanoparticle paste compositions from the vias prior to metalnanoparticle consolidation. In other embodiments, the removable supportmay be placed after filling of the vias but before significant sagginghas taken place. In some embodiments, the removable support may be analuminum foil or plate. KAPTON tape, graphoil or similarly performingmaterials may be used to provide sufficient support in some instances,as well as compressed, monolithic graphite plates or similar materials.The removable support may be removed from the face of the PCB followingconsolidation of the metal nanoparticles, or the removable support maydegrade, disintegrate, or otherwise undergo removal during the course ofconsolidating the metal nanoparticles (e.g., wax or thermally degradablepolymers). In more specific embodiments, the removable support may beused when vias having a diameter of about 5 mm or greater are beingfilled with a nanoparticle paste composition.

In some embodiments, a deadweight can be applied to the one or more viasafter filling, optionally in combination with a removable supportopposite the deadweight. The deadweight may aid in increasing compactionof metal nanoparticle paste composition prior to metal nanoparticleconsolidation, thereby densifying the monolithic metal body formed inthe vias. In some embodiments, the deadweight may be applied to the PCBfrom the face from which the vias are filled with the metal nanoparticlepaste composition.

In various embodiments, consolidating the metal nanoparticles maycomprise heating the metal nanoparticles above the fusion temperature,applying pressure to the metal nanoparticles, or any combinationthereof. Any heat source may be used to heat the metal nanoparticlesabove the fusion temperature, such as an oven, autoclave, heating tape,radiant heat source, laser, or the like. In some embodiments, directlaser sintering of the metal nanoparticles within the vias may takeplace. Other techniques for consolidating the metal nanoparticles withinthe vias may include, for example, applying pressure with a heatedpiston from one or both faces of the PCB to affect heating andcompaction simultaneously.

In some embodiments, the monolithic metal body within the vias may bemetallurgically bonded to a heat sink or similar thermal dissipationdevice. Direct metallurgical bonding to a heat sink may take place byplacing the heat sink in contact with the metal nanoparticle pastecomposition within the vias before metal nanoparticle consolidationtakes place. As such, when the metal nanoparticles undergo consolidationand bond with each other to form a monolithic metal body, the metalnanoparticles at the terminus of the via may similarly form ametallurgical bond to a heat sink in contact with the via as bulk metalis formed.

In some embodiments, the monolithic metal bodies within the vias may bebonded directly to the underside of a heat generating component usingthermal grease, a fused nanoparticle layer, solder, thermal underfill,or other methods common in the electronics industry for forming athermal interface. Thus, such thermal connections may further facilitateheat dissipation through the bottom face of the PCB. Alternately, themonolithic metal bodies may be directly metallurgically bonded to theunderside of a heat generating component.

Optionally, the walls of the vias may be plated with a metal prior tofilling the vias with the metal nanoparticle composition. The metalplating on the walls of the vias may promote adhesion of the PCB to themonolithic metal body following metal nanoparticle consolidation.Electrodeposition techniques, such as electroless plating, for example,may be used in particular embodiments to affect metal plating upon thewalls of the vias. The metal plating the walls of the vias may be thesame as or different than a metal comprising the monolithic metalbodies.

The printed circuit boards and methods of the present disclosure willnow be described with further reference to the drawings. FIG. 3A-3D showcross-sectional side views depicting various operations associated withfilling a plurality of vias in a printed circuit board with a metalnanoparticle paste composition and subsequently consolidating the metalnanoparticles. As shown, printed circuit board 100 includes substrate101, upon which conductive traces associated with a printed circuit andother components are defined (not visible in the cross-sectional sideviews of FIGS. 3A-3D). Vias 102 extend between at least top face 104 andbottom face 106 of substrate 101. Depending on particular applicationneeds, vias 102 may or may not be in electrical communication with theconductive traces or other components defining the printed circuit. Assuch, vias 102 may be functional, non-functional, or any combinationthereof with respect to operation of printed circuit board 100. Thus,vias 102 may or may not exhibit further functionality in addition toperforming their role in thermal management, as discussed furtherherein.

In FIG. 3B, vias 102 have been filled with metal nanoparticle pastecomposition 110. Optionally, a removable support and/or a deadweight(neither shown) may be utilized during or after filling to promoteretention of and/or densification of metal nanoparticle pastecomposition 110 within vias 102. In FIG. 3C, the metal nanoparticleswithin metal nanoparticle paste composition 110 have undergoneconsolidation to form monolithic metal body 120 that fills each of vias102. Each monolithic metal body 120 extends between at least top face104 and bottom face 106 with minimal void formation therein.

FIG. 3D shows a diagram of heat sink 130 placed in thermal contact withtop face 104 and heat sink 132 placed in thermal contact with bottomface 106. Heat transfer to heat sinks 130 and/or 132 may be facilitatedwith monolithic metal body 120 within each of vias 102. As mentionedabove, both of heat sinks 130 and 132 may be present, in someembodiments, and in other embodiments, only one of heat sinks 130 and132 may be present. Moreover, in some embodiments, one of heat sinks 130and/or 132 may be adjacent to top face 104 or bottom face 106 (e.g.,when directly contacting a heat-producing component of PCB 100) and theother of heat sinks 130 and 132 may directly contact vias 102 upon theopposite face. During direct contact, there is no additional thermalinterface layer intervening between monolithic metal body 120 and heatsink 132. Monolithic metal body 120 and heat sink 132 may comprise thesame metal in specific embodiments, such as both comprising copper. Whenmultiple heat sinks are present, the heat sinks may function by the sameor different mechanisms. In more specific embodiments, the multiple heatsinks may be both passive, both active, or a combination of active andpassive. In some embodiments, heat sink 132, for example, may comprise aportion of the mounting, strap, bracket, or case to which PCB 100 isaffixed.

In some embodiments, a bonding layer (not shown) may be disposed upontop face 104, which may allow monolithic metal bodies 120 to establish abond to the underside of a high-power electronic component (not shown).As such, the bonding layer allows thermal conduction to take placethrough monolithic metal bodies 120 through bottom face 106. That is,the bonding layer may promote heat transfer from the high-powerelectronic component to the opposite face of PCB 100. When a heat sinkalso directly contacts the high-power electronic component, dissipationof excess heat may take place from both faces of PCB 100. Alternately,monolithic metal bodies 120 may form a direct metallurgical bond to thehigh-power electronic component.

FIG. 4 shows a corresponding top view of printed circuit board 100,which shows an illustrative positioning of vias 102 and monolithic metalbodies 120. The location of vias 102 may vary depending upon thelocation of other components within or coupled to printed circuit board100, and the positioning of vias 102 should not be considered limiting.Moreover, the number of vias 102 should not be considered limiting, suchthat the number of vias 102 may be selected to promote a desired extentto thermal transfer. In the interest of clarity, additional electroniccircuitry and components of printed circuit board 100 are not shown inFIG. 4.

Embodiments disclosed herein include:

A. Thermal management methods employing one or more vias. The methodscomprise: providing a printed circuit board (PCB) defined upon asubstrate and having one or more vias extending through the substratebetween a first face and a second face; filling the one or more viaswith a metal nanoparticle paste composition that is adapted to limitshrinkage during metal nanoparticle consolidation; consolidating metalnanoparticles of the metal nanoparticle paste composition within the oneor more vias to form a monolithic metal body filling each of the one ormore vias; and placing a heat sink upon at least one of the first faceor the second face, the heat sink contacting each monolithic metal body.

B. Printed circuit boards comprising one or more thermal vias. Theprinted circuit boards comprise: a substrate having one or more viasextending therethrough between a first face and a second face; amonolithic metal body formed from metal nanoparticles filling each ofthe one or more vias, the monolithic metal body having a uniformnanoporosity ranging from about 2% to about 30%; and a heat sink upon atleast one of the first face or the second face, the heat sink contactingeach monolithic metal body.

C. Printed circuit boards comprising one or more thermal vias in excessof 1 mm in size. The printed circuit boards comprise: a substrate havingone or more vias extending therethrough between a first face and asecond face, at least a portion of the one of more vias being about 1 mmor larger in diameter; a monolithic metal body formed from metalnanoparticles filling each of the one or more vias; wherein at least aportion of the one or more vias contain a reinforcement material thatextends through at least a portion of the monolithic metal body; and aheat sink upon at least one of the first face or the second face, theheat sink contacting each monolithic metal body.

Each of embodiments A, B, and C may have one or more of the followingadditional elements in any combination:

Element 1: wherein the metal nanoparticles comprise coppernanoparticles.

Element 2: wherein the metal nanoparticle paste composition furthercomprises at least one additive selected from the group consisting ofdiamond particles, micron-scale metal particles, and any combinationthereof.

Element 3: wherein the monolithic metal body further comprises aplurality of diamond particles.

Element 4: wherein a first heat sink is located upon or adjacent to thefirst face of the PCB and a second heat sink is located upon or adjacentto the second face of the PCB.

Element 5: wherein the first heat sink and the second heat sinkdissipate heat by different mechanisms.

Element 6: wherein at least a portion of the one or more vias are filledusing a syringe that contains the metal nanoparticle paste composition.

Element 7: wherein at least a portion of the one or more vias are filledby screen printing the metal nanoparticle paste composition.

Element 8: wherein the method further comprises: placing a reinforcementmaterial in at least a portion of the one or more vias prior to fillingthe one or more vias with the metal nanoparticle paste composition.

Element 9: wherein the heat sink is placed in contact with the metalnanoparticle paste composition before consolidating the metalnanoparticles, and becomes metallurgically bonded to the monolithicmetal body as the metal nanoparticles undergo consolidation.

Element 10: wherein the monolithic metal body comprises copper and isformed from copper nanoparticles.

Element 11: wherein at least a portion of the one or more vias contain areinforcement material.

Element 12: wherein the heat sink is metallurgically bonded to themonolithic metal body.

Element 13: wherein the one or more vias are about 1 mm or larger indiameter.

Element 14: wherein at least a portion of the one or more vias are notin electrical communication with additional components located upon thePCB.

Element 15: wherein the nanoparticle paste composition has a densityranging from about 3.5 g/cm³ to about 6 g/cm³.

By way of non-limiting example, exemplary combinations applicable to A,B, C include: 1 and 2; 1 and 3; 1 and 4; 1, 4 and 5; 1 and 6; 1 and 7; 1and 8; 1 and 9; 1 and 10; 1, 10 and 11; 1 and 12; 1 and 13; 1 and 14; 1and 15; 2 and 4; 2, 4 and 5; 2 and 6; 2 and 7; 2 and 9; 2 and 10; 2 and11; 2 and 12; 2 and 13; 2 and 14; 2 and 15; 3 and 4; 3, 4 and 5; 3 and6; 3 and 7; 3 and 8; 3 and 9; 3 and 10; 3 and 11; 3, 10 and 11; 3 and12; 3 and 13; 3 and 14; 3 and 15; 6 or 7 and 8; 6 or 7 and 9; 6 or 7 and10; 6 or 7 and 11; 6 or 7 and 12; 6 or 7 and 13; 6 or 7 and 14; 6 or 7and 15; 10 and 11; 10 and 12; 10 and 13; 10 and 14; 10 and 15; 11 and12; 11 and 13; 11 and 14; 11 and 15; and 13 and 15.

Unless otherwise indicated, all numbers expressing quantities ofingredients, properties such as molecular weight, reaction conditions,and so forth used in the present specification and associated claims areto be understood as being modified in all instances by the term “about.”Accordingly, unless indicated to the contrary, the numerical parametersset forth in the following specification and attached claims areapproximations that may vary depending upon the desired propertiessought to be obtained by the embodiments of the present invention. Atthe very least, and not as an attempt to limit the application of thedoctrine of equivalents to the scope of the claim, each numericalparameter should at least be construed in light of the number ofreported significant digits and by applying ordinary roundingtechniques.

One or more illustrative embodiments incorporating the features of thepresent disclosure are presented herein. Not all features of a physicalimplementation are described or shown in this application for the sakeof clarity. It is understood that in the development of a physicalembodiment incorporating the present disclosure, numerousimplementation-specific decisions must be made to achieve thedeveloper's goals, such as compliance with system-related,business-related, government-related and other constraints, which varyby implementation and from time to time. While a developer's effortsmight be time-consuming, such efforts would be, nevertheless, a routineundertaking for those of ordinary skill in the art and having benefit ofthis disclosure.

Therefore, the present disclosure is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Theparticular embodiments disclosed above are illustrative only, as thepresent disclosure may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular illustrative embodiments disclosed above may be altered,combined, or modified and all such variations are considered within thescope and spirit of the present invention. The disclosure hereinsuitably may be practiced in the absence of any element that is notspecifically disclosed herein and/or any optional element disclosedherein. While compositions and methods are described in terms of“comprising,” “containing,” or “including” various components or steps,the compositions and methods can also “consist essentially of” or“consist of” the various components and steps. All numbers and rangesdisclosed above may vary by some amount. Whenever a numerical range witha lower limit and an upper limit is disclosed, any number and anyincluded range falling within the range is specifically disclosed. Inparticular, every range of values (of the form, “from about a to aboutb,” or, equivalently, “from approximately a to b,” or, equivalently,“from approximately a-b”) disclosed herein is to be understood to setforth every number and range encompassed within the broader range ofvalues. Also, the terms in the claims have their plain, ordinary meaningunless otherwise explicitly and clearly defined by the patentee.Moreover, the indefinite articles “a” or “an,” as used in the claims,are defined herein to mean one or more than one of the element that itintroduces.

The invention claimed is:
 1. A method comprising: providing a printedcircuit board (PCB) defined upon a substrate and having one or more viasextending through the substrate between a first face and a second face;filling the one or more vias with a metal nanoparticle paste compositionthat is adapted to limit shrinkage during metal nanoparticleconsolidation; consolidating metal nanoparticles of the metalnanoparticle paste composition within the one or more vias to form amonolithic metal body filling each of the one or more vias; and placinga heat sink upon at least one of the first face or the second face, theheat sink contacting each monolithic metal body.
 2. The method of claim1, wherein the metal nanoparticles comprise copper nanoparticles.
 3. Themethod of claim 2, wherein the metal nanoparticle paste compositionfurther comprises at least one additive selected from the groupconsisting of diamond particles, micron-scale metal particles, and anycombination thereof.
 4. The method of claim 1, wherein the metalnanoparticle paste composition further comprises at least one additiveselected from the group consisting of diamond particles, micron-scalemetal particles, and any combination thereof.
 5. The method of claim 1,wherein the monolithic metal body further comprises a plurality ofdiamond particles.
 6. The method of claim 1, wherein a first heat sinkis located upon or adjacent to the first face of the PCB and a secondheat sink is located upon or adjacent to the second face of the PCB. 7.The method of claim 6, wherein the first heat sink and the second heatsink dissipate heat by different mechanisms.
 8. The method of claim 1,wherein at least a portion of the one or more vias are filled using asyringe that contains the metal nanoparticle paste composition.
 9. Themethod of claim 1, wherein at least a portion of the one or more viasare filled by screen printing the metal nanoparticle paste composition.10. The method of claim 1, further comprising: placing a reinforcementmaterial in at least a portion of the one or more vias prior to fillingthe one or more vias with the metal nanoparticle paste composition. 11.The method of claim 1, wherein the heat sink is placed in contact withthe metal nanoparticle paste composition before consolidating the metalnanoparticles, and becomes metallurgically bonded to the monolithicmetal body as the metal nanoparticles undergo consolidation.
 12. Aprinted circuit board (PCB) comprising: a substrate having one or morevias extending therethrough between a first face and a second face; amonolithic metal body formed from metal nanoparticles filling each ofthe one or more vias, the monolithic metal body having a uniformnanoporosity ranging from about 2% to about 30%; and a heat sink upon atleast one of the first face or the second face, the heat sink contactingeach monolithic metal body.
 13. The PCB of claim 12, wherein a firstheat sink is located upon or adjacent to the first face and a secondheat sink is located upon or adjacent to the second face.
 14. The PCB ofclaim 12, wherein the monolithic metal body comprises copper and isformed from copper nanoparticles.
 15. The PCB of claim 14, wherein themonolithic metal body further comprises a plurality of diamondparticles.
 16. The PCB of claim 12, wherein the monolithic metal bodyfurther comprises a plurality of diamond particles.
 17. The PCB of claim12, wherein at least a portion of the one or more vias contain areinforcement material.
 18. The PCB of claim 12, wherein the heat sinkis metallurgically bonded to the monolithic metal body.
 19. The PCB ofclaim 12, wherein the one or more vias are about 1 mm or larger indiameter.
 20. A printed circuit board (PCB) comprising: a substratehaving one or more vias extending therethrough between a first face anda second face, at least a portion of the one of more vias being about 1mm or larger in diameter; a monolithic metal body formed from metalnanoparticles filling each of the one or more vias; wherein at least aportion of the one or more vias contain a reinforcement material thatextends through at least a portion of the monolithic metal body; and aheat sink upon at least one of the first face or the second face, theheat sink contacting each monolithic metal body.